Many people desire a longer operating lifetime for their battery-powered electronic, such as mobile telephones, MP3 players, and so forth. Typically in order for these battery-powered devices to conserve electrical power, their current consumption is managed preferably retaining operating characteristics thereof unaltered over as much of the battery operating parameters as possible. For example, many electrical devices operate in two well-known modes of operation—standby mode and operating mode. In the standby mode, minimal circuitry is enabled for reducing power consumption relative to the operating mode, where a majority of the circuitry is energized and the electrical device operates in accordance with its design parameters. Unfortunately, though this typical methodology is helpful, there is still a need to further conserve power.
Many of these battery operated electrical devices utilize power amplifier (PA) circuitry in order to amplify signals, such as for example RF signals. A very common PA output stage is an emitter follower circuit. Unfortunately, class A emitter follower stages typically have large quiescent currents resulting in significant power consumption regardless of input signal power and use. Thus, it is often these very PAs that are disabled in standby mode. Unfortunately, these PAs consume power due to the quiescent current whenever they are in the operating mode.
Ultimately, the quiescent current for a PA circuit for a given application is determined based on the amplification provided and the physical properties of the PA. Reducing the bias on a PA typically lessens the quiescent current and as such, reduces the overall power consumption. Of course, it also reduces the overall amplification. PAs are typically biased controlled in a “digital” or “analog” manner. In the digital control methodology switching of an effective bias voltage or current between discrete states is performed. In accordance with the analog methodology, a bias voltage is continuously varied using a voltage controlled biasing voltage or a voltage controlled current source. Since it would be advantageous to reduce quiescent current in order to reduce power consumption, this has been attempted and described in U.S. Pat. No. 5,825,228 which provides a low quiescent power, high output power rail-to rail amplifier output stage and U.S. Pat. No. 6,472,857 provide a very low quiescent current regulator circuit.
The challenge in using the prior art circuits arises when these circuits are used in current sensitive devices, such as those that are battery powered. One approach is to switch between different operational amplifier circuits each having a different topology and thereby switch between different amplification/quiescent current pairings. Although it has been readily demonstrated that implementing of a switching PA circuit is possible on an IC, it is an involved process to implement the actual control circuitry for controlling of the switching point for the PA. Such control circuitry for controlling of the switching requires a feedback loop with a form of calibration and detection algorithm along with external sensors and extensive considerations for handling hysteresis around the bias switching points. In addition, natural gain enhancements that occur with increasing bias level cause additional challenges for calibration approaches. This further complicates these devices because of the addition of more calibration points and results in increased testing time for assembled products. In practice, it is difficult to apply this switching technique because of changes in phase and gain when the RF path is switched cannot be tolerated by a baseband processor without data or call drops.
It is therefore an object of the invention to provide a mechanism for automatic control of quiescent current in PA circuits without employing the use of switching circuitry.